Evaluating the performance of refinery strippers

The performance of the strippers is of great importance for both process design and operation of refinery units. Loss of their performance can have adverse effects on the quality of the products. Vice versa, degradation of some important stripping related properties, such as flash point, may have nothing to do with the quality of stripping achieved by the process equipment. This article presents a simple criterion to tell whether inferior stripper performance is the culprit. The criterion is based on the gradient of the distillation curve of the stripped product under examination. The methodology can also be applied to process simulators to evaluate the effect of operating condition changes on the strippers’ performance.

Atmospheric crude oil distillation product stripping

The crude oil is fractionated to its products by the atmospheric distillation process. The raw material is preheated in a heat exchanger train, recovering heat from the unit’s products, and it is heated to a temperature of typically 365 - 370 °C in a fired heater. After that, it enters the flash zone of the atmospheric column. Part of the feed vaporises and flows through the rectifying section of the column, where it is split in its main products, typically, unstabilised naphtha, light and heavy kerosene, light and heavy gasoil.

Stripping quality

In order to evaluate the quality of each product’s stripping, one usually measures its flash point and compares it to the specification. Alternatively, one can check the 10% boiling point temperature of the ASTM D-86 distillation of the product and calculate the flash point using the API- 2B7 procedure. Generally, the lower the initial boiling point of the fraction, the worse the stripping quality. These criteria are able to tell one whether the product is on spec or not, but they cannot explain what causes the quality variation.

A criterion has been developed to discern whether the achieved product’s specification is due to better stripping or to worse fractionation. This criterion compares the gradients of the initial parts of the ASTM D-86 distillation curves of the products that are examined. The gradient is preferably computed for the 0 - 5% parts of the ASTM D-86 curves, which are the parts mainly expressing the influence of the light ends. In case the initial boiling point cannot be accurately determined, one can use the 5 - 10% part.

When these gradients differ less than ±1 °C/vol%, (i.e. the distillation curves are parallel), the stripping quality of the examined samples is the same. Any observed differences between the two product specifications have to be traced to different product yields. The stripping quality difference increases with the difference of the gradients. When the difference becomes greater than ±1 °C/vol%, the best quality corresponds to the distillation curve with the minimum light ends. This is the curve with the lower gradient.

Conclusion

A new criterion is introduced that is able to discern if the variations in the quality of the products of a crude distillation unit are due to different stripping quality, or other reasons. The criterion is simple and its parameters, the ASTM D-86 distillation curve gradients between the 0 - 5% or 5 - 10% points, can easily be calculated from lab data and compared with historical data. The criterion can also be applied to other standard types of distillation, such as ASTM D-1160 or TBP. Last but not least, it can be used in distillation column simulations in order to evaluate the performance of the strippers under changing process conditions.

Author: Constantinos Plellis-Tsaltakis and A.I. Lygeros, National Technical University of Athens.